1. Field of the Invention
This invention relates to a tantalum-niobium ore reduction process and, more particularly, to a process for obtaining metal rich in tantalum and niobium and lean in titanium from a titanium-rich tantalum-niobium ore.
2. The Prior Art
A number of refining methods for enriching the tantalum-niobium content of tantalum-niobium-lean ores and slags having TiO.sub.2 concentrations of up to 15% are known, and can be generally classified in two groups:
(I) Production of metal concentrates by mineral acid extraction; and
(II) Electrothermal ore reduction in the presence of carbon with subsequent wet-chemical purification of resulting carbides, followed by conversion of the carbides to tantalum-niobium oxide concentrates by oxidative roasting.
Methods typical of Group I are described in U.S. Pat. No. 3,972,710 (H. C. Starck Company), Austrian Pat. No. 315,512 (Union Carbide Corporation), U.S. Pat. No. 3,658,511 (Kawecki Berylco), and British Pat. No. 1,020,453 (Wah Chang).
In these processes, considerable quantities of relatively expensive mineral acids, such as hydrofluoric acid, are consumed. These processes are therefore expensive and require subsequent waste water treatment. Further, the presence of titanium oxides in the ore increases acid consumption and results in the formation of water-insoluble precipitates which are very difficult or impossible to remove from the reaction system by filtration. For these reasons, the Group I methods cannot be economically used for reduction of titanium-rich tantalumniobium ores, slags or residues.
Representative Group II methods are described in U.S. Pat. Nos. 3,811,867 (J. J. Scott), 3,721,727, 3,585,024 and 3,447,894 (Kawecki), and 2,972,530 (Zimmerley).
The initial step of each of the Group II processes is the formation of tantalum and niobium carbides and separation thereof from gangue by the melting of ores in an electric arc furnace, with the addition of carbon sources, in a reducing atmosphere. Depending on the gangue composition and the nature of the ore, temperatures of between 1200.degree. C. and 1800.degree. C. are required.
Titanium oxides are also converted to stable carbides under such carbothermal reduction conditions. Economical separation of tantalum and niobium carbides from the resulting titanium carbides is impossible. With ores having a titanium oxide content of more than 50%, as is the case in many naturally occurring ores having less than 20% (NbTa).sub.2 O.sub.5, furnace contents solidify during carbothermal reduction. Therefore, the methods of Group II are not suitable for separation of tantalum and niobium from carriers having a high titanium content.